Flash memory is used in a wide variety of electronic applications. Some flash memory cells utilize a floating gate field-effect transistor (FET), which stores one or more bits of data in the form of a variable level of electric charge within a “floating” gate. The floating gate is arranged above a channel of, and below a control gate of, the FET, but is electrically-isolated from both by an oxide layer. The memory cell stores the electric charge when the FET is in an “on” state (i.e., when current flows between the source and drain) by applying a voltage to the control gate, which causes electrons to tunnel from the channel into the floating gate. Because the floating gate is electrically-isolated from the channel and the control gate, electrons that tunnel into it will remain there indefinitely.
Electric charge trapped within the floating gate screens the electric field from the control gate within the channel, which increases the threshold voltage (Vt) of the FET. For flash memory devices that use an array of memory cells, the stored data can be read out of the array by measuring which cells experience Vt increase (e.g., store a “1”) and which don't (e.g., store a “0”).
Other flash memory cells utilize a charge-trapping memory layer, which includes a plurality of silicon nanocrystals, rather than a floating gate. Some flash memory cells with silicon nanocrystals tend to limit leakage, and hence improve memory data retention. This is because while a floating gate is a continuous layer, at least some of the silicon nanocrystals are spaced apart from one-another within the charge trapping layer. For example, the nanocrystals may tend to form “clumps” with small gaps between adjacent clumps. As a result, a leakage path from a single silicon nanocrystal, or single clump, results in only the loss of charge stored within that silicon nanocrystal, not the entire memory element.